In recent years, a semiconductor memory using a resistance change element as a storage element, such as a PRAM (phase-change random access memory) or an MRAM (magnetoresistive random access memory) has been attracting attention and been developed. The MRAM is a device which performs a memory operation by storing binary 1 or 0 in a memory cell by using magnetoresistance, and features nonvolatility, high-speed operation, high integration, and high reliability.
One of the magnetoresistive elements is a magnetic tunnel junction (MTJ) element including a laminated structure of three layers, namely, a storage layer having a variable magnetization direction, an insulating film as a tunnel barrier, and a reference layer maintaining a predetermined magnetization direction.
The resistance of the MTJ element varies with the magnetization directions of the storage layer and the reference layer, has a minimum value when the magnetization directions are parallel and has a maximum value when the magnetization directions are antiparallel, and stores information by associating the parallel state and the antiparallel state with binary 0 and 1.
There are schemes for writing information on the MTJ element: one is a magnetic field writing scheme in which only the magnetization direction of the storage layer is reversed by a current magnetic field generated when a current flows through a write line, and another is a writing scheme (of spin-injection) using spin angular momentum transfer in which the magnetization direction of the storage layer is reversed by passing a spin-polarized current through the MTJ element itself.
In the former scheme, when the element size is reduced, the coercivity of the magnetic body constituting the storage layer increases, and thus the write current tends to increase. Consequently, it is difficult to achieve both miniaturization and low current.
On the other hand, in the latter scheme (spin-injection writing scheme), the smaller the volume of the magnetic layer constituting the storage layer is, the fewer spin-polarized electrons will need to be injected. Therefore, it is expected that miniaturization and low current can both be easily achieved.